Conventionally, electro nickel plating has been popularly used for an ornamental purpose or for a corrosion prevention purpose, and is particularly used in the manufacture of battery cans, pipes and the like.
For example, a battery can is formed by drawing and, is usually manufactured through many steps using water-soluble emulsion as a lubricant for pressing. Here, it is necessary to apply a film which can withstand press forming to a surface of the battery can which constitutes an outer surface. Further, a problem which occurs during the formation of a battery can by press forming, is the lowering of a lifetime of a mold due to wear of the mold caused by the nickel powder in nickel plating. The problem largely influences the productivity of the battery can and hence, there has been a demand for a surface-treated metal sheet which is provided with a hard nickel plating film which does not generate nickel powder on a surface of a battery can which constitutes an outer surface.
For this end, as disclosed in patent document 1, there has been proposed a surface-treated metal sheet which can enhance surface hardness by firstly applying nickel plating to a metal sheet and, thereafter, by forming a tin-nickel diffusion layer on a nickel plating layer.
On the other hand, in preventing the corrosion of a steel substrate material or a zinc alloy substrate material by nickel plating having electrically noble potential more than these materials, when a pore exists in the nickel plating layer, the corrosion of the substrate material progresses. Accordingly, it is necessary for nickel plating to make the number of pores (pinholes) which reach the substrate material as small as possible. Although it depends on a state of a substrate material, to prevent the formation of pin holes in nickel plating, it is difficult to prevent lowering of corrosion resistance caused by pin holes in plating unless a use amount of a nickel plating weight is increased to a region where the manufacture of a surface-treated metal sheet is not economical. In view of the above, it is necessary for a nickel plated steel sheet which aims at the prevention of corrosion to reduce the number of pores in a nickel plated steel sheet as much as possible.
To cope with this situation, as disclosed in patent document 2, there has been proposed a method which forms a triion tetraoxide layer in a pin hole portion formed in plating by carrying out immersion treatment where a ferrous material is immersed into an alkali solution containing oxidizing agent such as sodium nitrite or potassium acetate after nickel plating or anode electrolytic treatment.